Calculate the moment of inertia of a rod whose linear density changes from `rho` to `etarho` from the thinner end to the thicker end. The mass of the rod is equal to M and length L. Consider the axis of rotation perpendicular to the rod and passing through the thinner end. Express your answer in terms of M,L and `eta` .
`[(1)/(6) ML^(2)(3eta+1)/(eta+1) ]`